last I was aware, the electrical residential wiring code called for only 3 wires, two main wires and a ground wire (often slightly smaller sized) so this is why everything 220v "inside" the house is running off or 6/2 with ground or 8/2 with ground romex.
for safety they use an extra main wire connected to the exterior panel ground lug is why you need 3 main wires when running under ground out to a shed or separate garage? its just extra safety in case there is a problem with the grounding rod for the shed or garage.
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